The development of engines over the past few years have included increasing the horsepower without increasing the displacement of the engine. To obtain this increased horsepower, it has been necessary to increase the combustion pressures of the combustion chamber which are transferred through the piston into the connecting rod.
Such increase in pressures have required the improvement of the integrity of the piston to withstand the increased stresses therein. In the past and in many cases today, such pistons are made of aluminum or cast iron. The piston described herein meets the requirement to withstand the increased pressures by providing a crown of a two piece articulated piston that is machined from a forging. The material of the forging is SAE4140, which is an alloy steel containing chrome and molybdenum.
During the development of the higher grade steel piston crown, the material was found to be extremely difficult to machine. Since the material is extremely tough, it has the characteristic of causing gumming of the tooling, and it was necessary to develop a unique machining method which is the subject of this invention.